The invention concerns a switchable tappet for a direct transmission of a cam lift to a tappet push rod in a valve train of an internal combustion engine, said tappet comprising a housing in whose axially extending cavity a longitudinally displaceable inner element is arranged that can be connected at least indirectly to the housing in a relative position to the housing by a coupling means, which housing comprises on an end facing away from the cavity, a contact surface for a cam, and which inner element comprises on a side of the cavity, a support for the tappet push rod while being biased away from the housing by at least one coiled compression spring, a hydraulic clearance compensation element comprising a pressure piston being installed in the tappet, which pressure piston defines on one end, a high pressure chamber, on an outer peripheral surface, a leak gap and with a bore, a reservoir for hydraulic medium.
A tappet of the pre-cited type is disclosed in DE 198 44 202 which is considered to be generic art. Due to the overlapping arrangement of a pressure piston of a hydraulic clearance compensation element in an inner element, which, in turn, extends in a housing, a leak gap from a high pressure chamber is formed axially below the pressure piston, which leak gap has an inadequate size because it is formed on a relatively small diameter. As a result, the hydraulic rigidity of the tappet is too low. In addition, it will be remarked that due to the arrangement of a coiled compression spring, also called a lost motion spring, in the interior, the overall length of the housing of the tappet is unnecessarily increased. Thus, this tappet has an unfavorable, large mass.
It is an object of the invention to provide a tappet of the pre-cited type in which the aforesaid drawbacks are eliminated by simple measures.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that the pressure piston is installed directly in the cavity of the housing, and the high pressure chamber is formed directly between an end of the pressure piston and the closed opposite end of the housing, the inner element extending directly in the bore of the pressure piston.
Due to the fact that the leak gap is shifted outwards as compared to the initially cited prior art, this leak gap is relatively large. The hydraulic rigidity of the tappet is thus markedly improved.
According to an important feature of the invention, an extension of the inner element projects beyond the cavity of the housing, and a coiled compression spring is arranged at least substantially around the extension while acting at one end on a support of the extension facing away from the reception, and at another end, on an edge or an edge-proximate section of the pressure piston or the housing.
Due to this shifting of the coiled compression spring to the outside of the housing, the height of this spring, that may also be a coiled compression spring assembly, is minimized. Thus, it is possible to replace relatively heavy housing sections with thin-walled components (extension of the inner element). In addition to the coiled compression spring arranged around the extension, if desired, a further coiled compression spring or another spring element can be arranged in the reservoir inside the pressure piston to enhance the action of the first-mentioned coiled compression spring. However, depending on the particular circumstances, it is also possible to use only one of the aforesaid coiled compression springs.
The support of the one end of the coiled compression spring around the extension of the inner element may have a one-piece or a multi-piece configuration. To mention just a few possibilities, this support may take the form of separate rings or locking rings or be made as a collar formed integrally on the extension. The support of the other end of the coiled compression spring is realized in a simple manner in that this spring is supported on an edge of the pressure piston or the housing.
As a coupling means, it is possible to use slides that, in an advantageous embodiment of the invention, are disposed in a radial web of the inner element and can be displaced radially outwards (in coupling direction) by the force of a compression spring, and radially inwards (in uncoupling direction), by hydraulic medium pressure. This radial web delimits in a simple manner with its underside, in a direction away from the high pressure chamber, a hydraulic medium reservoir that is enclosed by the pressure piston.
According to a simple measure of the invention for routing hydraulic medium to the clearance compensation element, the inner element, also for weight-saving reasons, has a hollow cylindrical configuration and, as viewed in the direction of flow, the support for the tappet push rod in the inner element has at least one opening through which the hydraulic medium can be transferred. Further, at least one passage is formed between a flank of the radial web and an inner peripheral surface of the inner element, through which passage, the hydraulic medium can be routed around the coupling means and further into the reservoir that is substantially enclosed by the pressure piston.
To prevent unnecessary splashing losses and the like in the uncoupled state of the inner element from the housing when the inner element sinks, at least one bridge member or a similar component having an opening for allowing the entry of hydraulic medium is arranged in the reservoir and limits the inner element in a direction towards the radial web. This bridge member can also have the function of an anti-leak device in inclined positions of the tappet.
According to another proposition of the invention, the extension for the coiled compression spring on the inner element is made of a light-weight material like sheet metal. If desired, this extension can also be made as a separate component and be connected to the inner element by appropriate connecting measures such as clipping, gluing, soldering or welding.
Finally, a particularly low-friction cam contact on the tappet is obtained if its contact surface is configured as a rotary roller that is advantageously mounted on the housing through a pin that may be supported on a rolling bearing if desired.